The present invention relates generally to gas turbine engines, and, more specifically, to turbines therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases which flow downstream through turbines that extract energy therefrom. High and low pressure turbines extract energy in turn for correspondingly powering the compressor and providing useful work, such as powering an upstream fan in an exemplary turbofan engine for powering an aircraft in flight.
In aircraft applications, engine weight and efficiency are primary design objectives for maximizing aircraft range and reducing operating costs. Weight and efficiency are interrelated in the various highly sophisticated components of the modern gas turbine engine built with precision typically measured in mils, and even fractions of mils.
Rotary turbine blades and stationary turbine nozzle vanes have corresponding airfoil profiles precisely configured for maximizing efficiency of energy extraction from the combustion gases. These components are heated by the hot combustion gases during operation, and therefore require suitable cooling for ensuring a long useful life.
Turbine airfoil cooling is conventionally effected by forming various cooling circuits therein which are fed from air bled from the compressor used as a coolant for protecting the airfoils during operation. Diverting compressed air from the combustor correspondingly decreases efficiency of the engine, and therefore it is desired to minimize the use of parasitic cooling air while suitably protecting the hot turbine components.
The prior art of turbine blades and nozzles is crowded with various forms of the cooling circuits therein and various forms of discharge cooling holes through the pressure and suction sidewalls of the airfoils.
Turbine airfoil discharge holes are found at various locations between the leading and trailing edges of the airfoil and from root to tip thereof, and have various sizes and configurations selected to improve performance in the complex three-dimensional combustion gas flow field surrounding the airfoils. And, the airfoils are subject to complex stress distribution at the different parts thereof due to the complex temperature distribution.
The cooling configurations of the turbine airfoils are also dependent on the physical size of the airfoils from large to small, with the smaller airfoils having additional problems for efficient cooling thereof due to the practical lower limit in size of cooling features which may be introduced therein.
In particular, the trailing edge of a turbine airfoil is relatively thin in view of the typical aerodynamic profile of the airfoil which has a maximum thickness near the leading edge, with the two sidewalls converging to the sharp trailing edge. The internal cooling circuit of the airfoil must correspondingly decrease in size to fit between the converging sidewalls of the airfoil, and typically must terminate before reaching the trailing edge for small airfoils, or airfoils having similarly thin trailing edges.
In order to adequately cool the airfoil trailing edge, the internal cooling circuit typically discharges into a row of trailing edge holes formed through the pressure side of the airfoil just upstream of the trailing edge. The discharged coolant provides a protective film of cooling air which travels downstream over the trailing edge for protection thereof from the hot combustion gases.
However, for particularly small turbine airfoils on the order of several centimeters in span height, the introduction of even the minimum-size outlet hole, on the order of about 10-15 mils in diameter, may require the placement of the trailing edge holes significantly upstream from the trailing edge itself, which decreases the cooling effectiveness of the discharged air.
Correspondingly, the minimum-size typical outlet holes may discharge more cooling air than required due to their relatively large size, and therefore decrease overall engine efficiency.
The ability to manufacture economically turbine airfoils is another significant objective in the design process. Turbine blades and vanes are typically cast using a ceramic core for the internal cooling features thereof in the conventional lost wax casting process.
Small features in the ceramic core correspondingly make the core fragile and subject to breaking during the manufacturing process which increases the overall cost of manufacture. Ceramic core yield is a significant factor in manufacturing turbine airfoils, and small cooling features embodied in the core are typically associated with lower yield.
For example, the trailing edge discharge holes are particularly problematic in manufacturing turbine airfoils since they correspondingly have small features which have practical lower-size limits in the casting of small turbine airfoils. The trailing edge holes have a finite axial or chordal length and are typically arranged in a radial row disposed in flow communication with a common radial flow passage inside the airfoil. The corresponding ceramic core has a common ceramic leg with a row of cantilevered ceramic fingers representing the trailing edge holes after casting.
For larger turbine airfoils, the ceramic core may be sufficiently strong for obtaining a sufficient yield for economically casting the airfoils. However, for small airfoils and small features the ceramic fingers would become unacceptably small and fragile leading to an unacceptable yield rendering their use in manufacture impractical.
Instead, the small airfoil may be cast with a corresponding ceramic core omitting the ceramic fingers and the trailing edge holes for casting the airfoil. The so-cast airfoil then undergoes a subsequent manufacturing operation for drilling the trailing edge holes, typically using electrical discharge machining (EDM) or electrostream (ES) machining for achieving the small size and tolerances required for the small trailing edge holes.
Accordingly, it is desired to provide a turbine airfoil having an improved trailing edge cooling design for increasing cooling efficiency in a configuration which may be manufactured using a corresponding ceramic core with suitable yield, even for typically small turbine airfoils.